An assimilatory sulfite reductase, CysI, negatively regulates the dormancy of Microbulbifer aggregans CCB-MM1.

Sulfur is one of the common and essential elements of all life. Sulfate, which is a major source of sulfur, plays an important role in synthesizing sulfur-containing amino acids, such as cysteine and methionine, organic compounds essential to all living organisms. Some investigations reported that the assimilatory sulfate reduction pathway (ASRP) involved in cysteine synthesis is crucial to entering bacterial dormancy in pathogens. Our previous investigation reported that the halophilic marine bacterium, Microbulbifer aggregans CCB-MM1 , possesses an ASRP and the dissimilatory sulfate reduction pathway (DSRP). The bacterium might use DSRP to generate energy required for entering its dormant. However, the role of the ASRP in the dormancy of M. aggregans CCB-MM1 was so far unknown. In this study, we found that genes involved in ASRP were downregulated in the dormancy. The disruption of the gene encoding an assimilatory sulfite reductase, cysI, suppressed a completely dormant state under low nutrient conditions. In addition, the cysI mutant showed cell aggregation at the middle-exponential phase under high nutrient conditions, indicating that the mutation might be stimulated to enter the dormancy. The wild-type phenotype of the bacterium was recovered by the addition of cysteine. These results suggested that cysteine concentration may play an important role in inducing the dormancy of M. aggregans.